Numerical Simulation On Sapphire Growth Process With Self-regulating Thermal Boundary Condition Method

Sapphire crystal material has been widely used in aviation and space industries due to its impressive comprehensive properties.As modern science and technique develops,the size,the quality and the capacity of sapphire crystal cannot satisfy the demand of modern industries.Therefore,proposing a new method making automatic production of sapphire crystal with large size and high quality has been an important issue.In this thesis,sapphire growth furnaces with Kyropoulos method are simplified reasonably.The growth process of sapphire crystal with seven different sizes is investigated using Finite Element Method.Then,the temperature distribution,the temperature gradient distribution at solid-liquid interface,solid-liquid interfacial angle and Von Mises stress distribution of sapphire crystal at each growth stage are simulated by regulating thermal boundary conditions.Simulation results,which are the evaluation norms of sapphire crystal quality,can also help to regulate the thermal boundary conditions in turn.Finally,iteratively refined thermal boundary conditions are obtained to grow sapphire crystal with high quality by acquiring accurate amount of energy in each period of crystal growth.In seeding stages,the temperature of furnace remains the same.The growth of crystal seed can be controlled by simulating the power of heat exchanger with micro-pulling of crystal seed.In shouldering stages,the growth of crystal shoulder can be controlled by simulating the power of heat exchanger cooperated with decreasing wall temperature.In iso-diameter stage,the growth of sapphire crystal is controlled mainly by decreasing the furnace wall temperature.In ending stage,the temperature of furnace walls decreases more slowly to decrease defect value.The convective heat transfer coefficient decreases simultaneously to eliminate the thermal stress inside the sapphire crystal.According to simulation results of the temperature distribution,the temperature gradient distribution,the interfacial angle and the thermal stress distribution,the proper thermal boundary conditions at each stage are acquired.Based on above,a new crystal method called self-regulating method,which concentrates on regulation techniques to guarantee the crystal acquiring accurate thermal amount at each growth stage has been proposed.In this research,the thermal field's influence induced by thermal boundary conditions of sapphire furnaces is discussed,instead of furnace structure and insulation equipment.In actual production of sapphire crystal,to guarantee the quality of sapphire crystal,the designed furnace structure and energy input are required to match the thermal boundary conditions.This method is expected to offer some theoretical guidance to make automatic production of large-size sapphire crystal available.